Method for providing information in a cellular wireless communication system

ABSTRACT

The present disclosure relates to a method for providing information in a cellular wireless communication system including: detecting a radio link failure (RLF) for a mobile station while connected to a first cell; re-establishing the connection in a second cell; and providing information about the radio link failure (RLF) only to cells supporting the same radio access technology (RAT) as the first cell and/or a third cell, wherein the third cell is the cell to which the mobile station was connected before the first cell. Furthermore, the disclosure also relates to a method in a mobile station, a method in a base station, a computer program, a computer program product, a mobile station device and a base station device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2010/075983, filed on Aug. 13, 2010, which is hereby incorporatedby reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a method for providing information ina cellular wireless communication system, or more particularly to amethod according to the preamble of claim 1. Furthermore, the disclosurealso relates to a method in a mobile station, a method in a basestation, a computer program, a computer program product, a mobilestation device and a base station device.

BACKGROUND

Normally, a mobile station (MS) in active mode in a cellular wirelesscommunication system is handed over from one cell to the next as itmoves through the system, and data can be transmitted and receivedwithout significant interruptions due to these handovers.

A handover (HO) procedure can consist of many steps. In many cellularwireless communication systems a HO is: 1) network controlled, i.e. theMS is commanded by the network when to connect to another cell; 2)prepared, i.e. the target cell to which the MS is moving to is prepared;and 3) MS assisted, i.e. the MS provides measurement reports before HOto the serving cell to assist the decision to do HO preparation oftarget cell(s), and when to leave the serving cell/connect to the targetcell.

In the context of HO, the serving cell before HO is often referred to asthe source cell. After successful HO the target cell becomes the newserving cell. In Long Term Evolution (LTE) the HO is a “hard handover”,which means that the UE radio link is switched from one (source) cell toanother (target) cell. In Universal Mobile Telecommunications System(UMTS) hard handovers are used exclusively for Time Division Duplex(TDD) mode, but may also be used for Frequency Division Duplex (FDD)mode.

In the following discourse, the focus is on the intra freq LTE HOprocedure, but the procedures are similar for the LTE inter Radio AccessTechnology (RAT) and LTE inter frequency HO procedures. The intraE-UTRAN in RRC_CONNECTED state is a User Equipment (UE) assisted networkcontrolled HO, with HO preparation signalling in E-UTRAN.

A HO is initially triggered by a measurement report sent from a UE to aserving eNB (E-UTRAN NodeB). The serving eNB configures how the UE shalltake measurements, and under what conditions a measurement report shallbe triggered and sent to the eNB.

To assist mobility control decisions, the UE can measure severaldifferent cells and report the results to the network. Differentnetworks and network deployments can have different detailed behaviour,but in most systems it is natural to trigger HO when signal receptionfrom a target cell is better than from a source cell.

For the case of intra-frequency HO in a reuse-one system (i.e. in asystem where the source cell and the target cell uses exactly the samefrequency resources) there are strong interference management benefitsin (always) keeping the UE connected to the cell with the best signalstrength. In the measurement report, the UE includes the reason for thetrigger of a HO, e.g. target cell signal stronger than serving cellsignal, and measurements of a Reference Signal Received Power (RSRP) orReference Signal Received Quality (RSRQ) of the serving cell and severalneighbour cells including the target cell. To reduce ping-pong effectswhere a UE is handed over repeatedly between two cells a HO offset isoften added to the HO trigger condition: target cell signal should bebetter than the serving cell signal by an offset, wherein the offsetvalue >0 dB.

When a serving eNB receives a measurement report from a UE and if theeNB wishes to HO the UE to another cell, the eNB performs a HOpreparation to that cell. HO preparation involves a signalling exchangebetween one (serving) eNB and another (target) eNB. The source cellrequests the HO (Handover Request) and passes over UE contextinformation; and the target cell decides if it can admit the UE (CallAdmission Control) and either accepts or rejects the HO. In anacceptance message (Handover Request Ack), the target cell includesparameters required by the UE to allow it to communicate to the targetcell—these parameters being grouped into a transparent container. Thesource cell may prepare multiple cells for HO.

Following a successful preparation, the HO execution takes place. Thesource cell issues a HO Command to the UE—this is theRRCConnectionReconfiguration message and carries the transparentcontainer. If, and when, the UE receives this message correctly the UEsynchronises to the new target cell and sends a synchronisation messageon the Random Access Channel (RACH). The target cell then issues anallocation to the UE so that the UE can send a HO confirmation messageto the target cell (RRCConnectionReconfiguration-Complete message).

In the final steps (Handover Completion), which do not involve the UE,the source eNB (serving the source cell) is able to forward data(un-acknowledged downlink packets) to the target eNB (serving the sourcecell), and the S1-U interface from the Serving Gateway (S-GW) must beswitched from the source to the target cell (“path switch”). Finally, ifthe handover is successful, the target eNB issues a UE Context Releasemessage to the source eNB.

However, it is possible for a HO to fail at different points because ofa Radio Link Failure (RLF) or a failure by the RACH. A RACH failureduring a HO is called “Handover Failure” in 3GPP TS36.331, but for theremainder of this disclosure the term HO failure is used to compriseboth RLF and RACH failures.

After a HO failure, the UE attempts a RRC re-establishment which isdescribed in specifications 3GPP TS36.300 and 3GPP TS36.331. The UEfirstly tries to find the strongest cell that it can detect (“cellselection”), and then the UE sends aRRCConnection-ReestablishmentRequest to the cell that it has selected.If this selected cell has prior knowledge of the UE and detailsregarding the UE connection (e.g. security parameters, this is calledthe “UE Context”) then the cell can send a RRCConnectionReestablishmentand the re-establishment will succeed which means that the UE remains inRadio Resource Control (RRC) connected state.

If however the UE context is lacking, the re-establishment request isrejected and the UE drops to RRC idle state, which results in furtherdelay before the UE can transit to RRC connected state and recommenceany data communication. The “UE Context” may be passed to a cell duringthe HO procedure or at some other point in time. This transfer is calledHO preparation. Note also that the RRCConnectionReestablishmentRequestcarries three fields, the Cell Radio Network Temporary Identifier(C-RNTI) of the UE in the serving cell where failure occurred, thePhysical Cell Identity (PCI) of this cell, and the shortMAC-I calculatedusing the Identity (ID) of the re-establishment cell.

The hard HO in the Universal Mobile Telecommunications System (UMTS) isvery similar in many respects to the above description—i.e. also beingUE assisted but network controlled, which means that the UE isconfigured to send triggered measurement reports but the network decideswhen to execute the HO; exploits preparation (using Radio Link Setupprocedure); is “backward” HO, which means that the source cell sends theHO command to the UE and the UE replies to the target cell; and iscompleted by inter-node signalling.

Furthermore, RLF is described in specifications 3G PP TS36.300 and 3GPPTS36.331. One form of RLF is driven by out-of-sync detection by Layer 1.A radio problem detection procedure is started when a UE receives acertain number of consecutive “out-of-sync” indications from lowerlayers. The number of consecutive indications is specified by thethreshold N310. When this happens, the UE starts a timer T310. In casethe UE receives a certain (N311) consecutive “in-sync” indications fromlower layers while T310 is running, the UE shall stop the timer andreturn to normal operation.

Following the declaration of a RLF, the UE attempts cell selection. Ifthe UE manages to find a cell to connect to within the cell selectionphase, the UE will attempt to re-establish RRC to this cell. If, on theother hand, the UE does not find a cell within the cell selection phase(T311), the UE goes back to idle mode and may start looking for cells onother RATs, examples of which are LTE, UMTS, WiMaX and GSM EDGE RadioAccess Network (GERAN).

RLF can also be declared by the Radio Link Control (RLC) layer of the UEwhen a maximum number of transmissions have been reached fortransmission of an uplink RRC signalling packet, but the packet hasstill not been delivered successfully. Additionally, if the randomaccess during the HO fails (T304 timeout) the UE behaves as if a RLF hadoccurred. In the present discourse the term RLF relates to any of theabove mentioned events.

Moreover, a RLF report was introduced in 3GPP Rel-9 to enable a eNBreceiving a RLF report to distinguish between Mobility RobustnessOptimisation (MRO) related problems and coverage problems. This was doneby including a set of neighbour cell measurements indicating the signalstrength at the time of failure. With the help of this, the eNB is ableto see if there is an alternative neighbour cell that might have beenused, or if there is no neighbour detected in the case of a coveragehole.

The RLF report carries information about:

Serving cell RSRP, and optionally RSRQ;

Neighbours cell RSRP/RSRQ; and

May also indicate the strength of detected inter-RAT neighbour cells.

In Rel-9, if a RLF during a HO is followed by a successful RRCRe-establishment, it is possible to include a RLF Report in a RLFINDICATION message that is sent from a eNB where re-establishment takesplace to a eNB that was serving the UE at the point of RLF. Thecapability of the UE to provide the RLF Report is indicated by a flag inthe RCConnectionReestablishmentComplete message. The RLF Report is thenprovided to the eNB where re-establishment took place using the UEInformation procedure.

One remaining problem in Rel-9 is that the UE is only able to send a RLFreport if RRC re-establishment is successful. And this is only possibleif the cell receiving the RLF report has the context of this UE (it is“prepared” for the HO). In most HO failure cases, the HO is executed toolate and the cell where the UE attempts RRC re-establishment is notprepared, so the UE can not send the RLF report.

In order to mitigate this, there have been suggestions to allow the RLFreporting to take place also after the UE has gone back to idle mode.This would mean that the UE reports when attempting RRC establishment.

Furthermore, in 3GPP there has been considerable study intoSelf-Organising

Networks (SON) for LTE. One part of this is the Handover ParameterOptimisation also known as the above mentioned MRO which is aiming atoptimising mobility parameters. It has not been specified which 1-10parameters shall be optimised, but examples include the HO hysteresis(also called offset) and the Time to Trigger (TTT) parameters. The aimsof the optimisation are to reduce HO failures whilst at the same timenot having more HOs than are necessary. The MRO functionality isdistributed in the Evolved-UTRAN (E-UTRAN), i.e. every eNB has its ownMRO optimisation function. To assist optimisation, signalling has alsobeen defined between eNBs to help identify HO failure events.

The following is the text describing the use-case of Handover ParameterOptimisation also known as MRO in section 22.5 of specification 3GPPTS36.300, 9.2.0:

-   -   One of the functions of Mobility Robustness Optimization [MRO]        is to detect RLFs that occur due to Too Early or Too Late        Handovers, or Handover to Wrong Cell. This detection mechanism        is carried out through the following procedures:        -   [Too Late HO] If the UE attempts to re-establish the radio            link at eNB B after a RLF at eNB A then eNB B may report            this RLF event to eNB A by means of the RLF Indication            Procedure.        -   [Too Early HO] eNB B may send a HANDOVER REPORT message            indicating a Too Early HO event to eNB A when eNB B receives            an RLF Indication from eNB A and if eNB B has sent the UE            Context Release message to eNB A related to the completion            of an incoming HO for the same UE within the last            Tstore_UE_cntxt seconds.        -   [HO to Wrong Cell] eNB B may send a HANDOVER REPORT message            indicating a HO To Wrong Cell event to eNB A when eNB B            receives an RLF Indication from eNB C, and if eNB B has sent            the UE Context Release message to eNB A related to the            completion of an incoming HO for the same UE within the last            Tstore_UE_cntxt seconds. The indication may also be sent if            eNB B and eNB C are the same and the RLF report is internal            to this eNB.    -   The detection of the above events is enabled by the RLF        Indication and Handover Report procedures.    -   The RLF Indication procedure may be initiated after a UE        attempts to re-establish the radio link at eNB B after a RLF at        eNB A. The RLF INDICATION message sent from eNB B to eNB A shall        contain the following information elements:        -   Failure Cell ID: PCI of the cell in which the RLF occurred;        -   Reestablishment Cell ID: ECGI of the cell where RL            re-establishment attempt is made;        -   C-RNTI: C-RNTI of the UE in the cell where RLF occurred.        -   shortMAC-I (optionally): the 16 least significant bits of            the MAC-I calculated using the security configuration of the            source cell and the re-establishment cell identity.    -   eNB B may initiate RLF Indication towards multiple eNBs if they        control cells which use the PCI signalled by the UE during the        re-establishment procedure. The eNB A selects the UE context        that matches the received Failure cell PCI and C-RNTI, and, if        available, uses the shortMAC-I to confirm this identification,        by calculating the shortMAC-I and comparing it to the received        IE.    -   The Handover Report procedure is used in the case of recently        completed handovers, when an RLF occurs in the target cell (in        eNB B) shortly after it sent the UE Context Release message to        the source eNB A. The HANDOVER REPORT message contains the        following information:        -   Type of detected handover problem (Too Early HO, HO to Wrong            Cell)        -   ECGI of source and target cells in the handover        -   ECGI of the re-establishment cell (in the case of HO to            Wrong Cell)        -   Handover cause (signalled by the source during handover            preparation)

Inter RAT Mobility

HO between different RATs, i.e. inter RAT HO, can have different causes.Two examples could be:

-   -   Coverage—the coverage on a current RAT is not sufficient, but        there exist coverage on other RATs. Therefore, the UE may be        ordered to HO to another RAT;    -   Capacity—the capacity in the current RAT may not be sufficient,        but there exist available capacity in another RAT covering the        same area.

For inter RAT mobility based on coverage there is typically two absolutesignal strength, or signal quality thresholds: criteria in source RATand target RAT. The target RAT criterion can be used to set thethreshold for which a UE in a LTE system is expected to survive in thetarget RAT. The source criterion can be used to adjust at what time theUE starts performing inter RAT measurements.

Using a too low criterion for the serving cell leads to excessivemeasurements and using a too high criterion may lead to dropped callssince the UE is not able to find an alternative before the quality tothe serving cell is too poor to use for further communication.

Typical events available for measurement configuration according tospecification 3GPP TS36.331 is:

-   -   Event A1: Serving becomes better than threshold.    -   Event A2: Serving becomes worse than threshold.    -   Event A3: Neighbour becomes offset better than serving.    -   Event A4: Neighbour becomes better than threshold.    -   Event A5: Serving becomes worse than threshold1 and neighbour        becomes better than threshold2.    -   Event B1: inter RAT neighbour becomes better than threshold.    -   Event B2: Serving becomes worse than threshold1 and inter RAT        neighbour becomes better than threshold2.

Here “serving” refers to a serving or source cell, and “neighbour”refers to a detected neighbour cell. The different events compare signalstrength (or quality) of cells against fixed thresholds, or the valuesof serving and neighbour cells are compared (including a hysteresisvalue as described above).

One typical implementation of an inter RAT HO algorithm (coverage HO)would be as follows:

-   -   Inter RAT measurements are started when the quality of the        serving cell is below an acceptable level, using event A2.    -   At that time measurements gaps are configured (if needed) and        the UE is configured to report cells from another RAT using        event B1 or B2

Inter RAT MRO

An inter RAT MRO functionality in a eNB could adjust the followingparameters:

-   -   When measurement on the other RAT cells starts (A2) and when        report should be triggered (B2_threshold1));    -   Requirements on the target cell before reporting (B1 and        B2_threshold2);    -   Setting of different offsets for different frequencies (the        offset is set via parameter ofn).

The error cases for inter RAT MRO is slightly different compared to MROfor HOs within LTE (intra-frequency or inter-frequency handover).Different error cases could be, where IRAT denotes inter RAT:

-   -   IRAT too late—the threshold requirements for serving cell is set        too low, causing the UE to move out of the serving cell before        the measurement reports can be started, or the HO can be        executed;    -   IRAT too early—the threshold requirements on the target cell are        too low, causing the HO to fail, or an RLF occur shortly after        HO;    -   IRAT wrong RAT—RLF occurring shortly after an HO to another RAT        leading to a re-establishment in a third RAT;    -   IRAT frequent—the requirements on the serving cell is set too        strict, casing inter RAT HO even if an intra LTE HO would have        been possible;    -   IRAT ping pong—the requirement on the target cell is too low        compared to the requirement on the source cell when the UE has        changed RAT;    -   IRAT rapid HO—Rapid HO to another RAT occurring after HO.

It may be assumed that the “IRAT too late” is the most important failurecase, at least initially, since the LTE system may have spotty coverageand will suffer if a HO to legacy systems is not performed in time. Theoccurrence of “IRAT too late” can be measured by counting the number oftimes a UE encounter a RLF in the cell, goes back to idle mode andmanages to find a new cell in another RAT.

There has been a proposal to extend the existing intra LTE MRO to enablenetwork nodes from different RATs to exchange information (e.g. NGMNAlliance, Handover Optimisation). In the “IRAT too late” example, thiswould mean that the cell where the UE manages to connect after the RLFwill receive a report from the UE with details on the serving cell priorto the failure. This eNB would then need to send a message similar to“RLF indication” to the eNB handling the cell where the UE was connectedto before the RLF. It has been discussed that the RAN InformationManagement (RIM) interface should be used to enable this informationtransfer between different RATs.

In order to collect and exchange information across RATs, the differentRATs have to be able to decode information received from the UE,concerning another RAT and also send this information in a commonlyagreed format between the RATs.

Further, the network interface most probable to be used is the RIM. Thishas been agreed to be used for other purposes for communication betweendifferent RATs, with the requirement that the signalling should belimited. The reason for this limitation is the concern about processingcomplexity for core network nodes. Hence it could be problematic alsousing this interface for inter RAT MRO.

There is therefore a need for a method for providing information in acellular wireless communication system mitigating or solving theproblems of prior art.

SUMMARY

The object of the present disclosure is to provide a method whichmitigates or solves the problems of prior art. Another object of thedisclosure is to provide a method which improves detection of inter RATHO problems, reduces signalling, and is compatible with existingcellular communication systems. A yet another object of the disclosureis to provide an alternative method for providing information in acellular wireless communication system.

According to one aspect of the disclosure, the objects are achieved witha method for providing information in a cellular wireless communicationsystem, wherein each cell in said cellular wireless communication systemis served by a base station and supports a radio access technology (RAT)for radio communication between a cell and one or more mobile stationsconnected to said cell; said cellular wireless communication systememploying a procedure in which a mobile station may be handed over froma cell to another cell supporting different radio access technologies(RATs), and further employing a procedure in which a mobile stationsuffering from a radio link failure (RLF), when being connected to acell, may attempt to re-connect to another cell supporting a differentradio access technology (RAT), comprising the steps of:

detecting a radio link failure (RLF) for a mobile station whileconnected to a first cell;

re-establishing the connection in a second cell; and

providing information about said radio link failure (RLF) only to cellssupporting the same radio access technology (RAT) as said first celland/or a third cell, wherein said third cell is the cell to which saidmobile station was connected before said first cell.

Embodiments of the method in a cellular wireless communication systemabove are disclosed in the dependent claims 2-16.

According to another aspect of the disclosure, the objects are alsoachieved with a method in a mobile station for providing information ina cellular wireless communication system, wherein each cell in saidcellular wireless communication system is served by a base station andsupports a radio access technology (RAT) for radio communication betweena cell and one or more mobile stations connected to said cell; saidcellular wireless communication system employing a procedure in which amobile station may be handed over from a cell to another cell supportingdifferent radio access technologies (RATs), and further employing aprocedure in which a mobile station suffering from a radio link failure(RLF), when being connected to a cell, may attempt to re-connect toanother cell supporting a different radio access technology (RAT),comprising the steps of

detecting a radio link failure (RLF) while being connected to a firstcell;

re-establishing the connection in a second cell; and

providing information about said radio link failure (RLF) only to cellssupporting the same radio access technology (RAT) as said first celland/or a third cell, wherein said third cell is the cell to which saidmobile station was connected before said first cell.

According to yet another aspect of the disclosure, the objects are alsoachieved with a method in a base station for providing information in acellular wireless communication system, wherein each cell in saidcellular wireless communication system is served by a base station andsupports a radio access technology (RAT) for radio communication betweena cell and one or more mobile stations connected to said cell; saidcellular wireless communication system employing a procedure in which amobile station may be handed over from a cell to another cell supportingdifferent radio access technologies (RATs), and further employing aprocedure in which a mobile station suffering from a radio link failure(RLF), when being connected to a cell, may attempt to re-connect toanother cell supporting a different radio access technology (RAT),comprising the steps of:

receiving a radio link failure (RLF) report relating to a radio linkfailure (RLF) for a mobile station while said mobile station isconnected to a first cell; and

providing information about said radio link failure (RLF) to said firstcell and/or to a third cell if said base station supports the same radioaccess technology (RAT) as said first cell or said third cell, whereinsaid third cell is the cell to which said mobile station was connectedbefore said first cell.

According to an embodiment of the method in a base station above, saidstep of providing information involves:

transmitting said information to a first base station serving said firstcell, or to a third base station serving said third cell, directly orvia one or more X2 and/or S1 interfaces over one or more other basestations in said cellular wireless communication system.

The method in a mobile station and in a base station above may bemodified according to different embodiments of the method in a cellularwireless communication system.

The disclosure also relates to a computer program and a computer programproduct when run in a computer causes the computer to execute the methodin a mobile station and the method in a base station described above.

According to another aspect of the disclosure, the objects are alsoachieved with a mobile station device for providing information in acellular wireless communication system, wherein each cell in saidcellular wireless communication system is arranged to be served by abase station and supports a radio access technology (RAT) for radiocommunication between a cell and one or more mobile stations connectedto said cell; said cellular wireless communication system employing aprocedure in which a mobile station may be handed over from a cell toanother cell supporting different radio access technologies (RATs), andfurther employing a procedure in which a mobile station suffering from aradio link failure (RLF), when being connected to a cell, may attempt tore-connect to another cell supporting a different radio accesstechnology (RAT); being configured to:

detect a radio link failure (RLF) while being connected to a first cell;

re-establish the connection in a second cell; and

provide information about said radio link failure (RLF) only to cellssupporting the same radio access technology (RAT) as said first celland/or a third cell, wherein said third cell is the cell to which saidmobile station was connected before said first cell.

According to another aspect of the disclosure, the objects are alsoachieved with a base station device for providing information in acellular wireless communication system, wherein each cell in saidcellular wireless communication system is arranged to be served by abase station and supports a radio access technology (RAT) for radiocommunication between a cell and one or more mobile stations connectedto said cell; said cellular wireless communication system employing aprocedure in which a mobile station may be handed over from a cell toanother cell supporting different radio access technologies (RATs), andfurther employing a procedure in which a mobile station suffering from aradio link failure (RLF), when being connected to a cell, may attempt tore-connect to another cell supporting a different radio accesstechnology (RAT), being configured to:

receive a radio link failure (RLF) report relating to a radio linkfailure (RLF) for said mobile station while said mobile station isconnected to a first cell; and

provide information about said radio link failure (RLF) to said firstcell and/or to a third cell if said base station supports the same radioaccess technology (RAT) as said first cell or said third cell, whereinsaid third cell is the cell to which said mobile station was connectedbefore said first cell.

The mobile station device and base station device according to thedisclosure may also be arranged according to the different embodiment ofthe methods above.

By using the present solution, it is possible to support inter-RAT MROwithout having to introduce signalling across different RATs and withouthaving to define mechanisms for how a UE shall be able to report theoccurrence of a RLF occurring in a different RAT.

The disclosure further provides the advantages effects of: enabling bothdetection of inter-RAT RLF problems, e.g. relating to handover problems,and signalling of this knowledge to an original source cell wherecorrective actions may be taken to avoid the same problem with futureinter-RAT handovers; addresses a number of inter-RAT handover failurecases, in particular “IRAT too late”, “IRAT too early”, “IRAT wrongRAT”; only impacts the LTE signalling functionality of the UE and theeNB; it avoids any signalling from one RAT to another, for example,using the RIM procedure, thereby avoiding additional loading on the corenetwork elements of both RATs; has a smaller impact on the 3GPPstandardisation specifications than the prior art solutions; onlyimpacts LTE standardisation specifications; may re-use existingprocedures over the X2 interface, such as RLF indication message orHandover report message, with suitable extensions; and is applicable tohandovers to non-3GPP RATs, such as WiMax or Wi-Fi.

Other advantages and applications of the present disclosure will beapparent from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings are intended to clarify and explain differentembodiments of the present disclosure in which:

FIG. 1 shows a typical cell scenario with cell measurement;

FIG. 2 shows an exemplary embodiment of the disclosure in the IRAT toearly and to late cases; and

FIG. 3 shows an exemplary embodiment of the disclosure in the IRAT towrong RAT case.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

With reference to the discussion above, the present disclosure providesa method to identify inter-RAT RFL problems, e.g. HO problems, andforward this information to an original source cell.

Therefore, the disclosure relates to a method for providing informationin a cellular wireless communication system, wherein each cell in saidcellular wireless communication system is served by a base station (BS),which means that each cell comprises a BS serving that cell. Further,each cell supports a RAT for radio communication between a cell and oneor more MSs connected to the cell. The system employs a procedure inwhich a MS may be handed over from a cell to another cell supportingdifferent RATs. The system further employs a procedure in which a MSsuffering from a RLF, when being connected to a cell, may attempt tore-connect to another cell supporting a different radio accesstechnology (RAT) than the RAT supported by said cell.

The method comprises the steps of: detecting a RLF for a MS when the MSis connected to a first cell; re-establishing the broken connection in asecond cell; and providing information about the RLF only to cellssupporting the same RAT as the first cell and/or a third cell. The thirdcell is the cell to which the MS was connected before the MS connectedto the first cell.

The present disclosure therefore involves, according to an embodiment, amechanism in which the MS determines the source cell in case a RLFfailure occurs and reports this only to a cell belonging to the same RATas the source cell belongs to.

Thereby, the need for signalling is reduced between different RATs, andfurther there is no need to translate information between differentRATs. Hence, according to an embodiment of the disclosure, the MS sendsa RLF report when returning to the same RAT as the source cell.

According to another embodiment of the disclosure, the RLF reportincludes one or more in the group comprising: measurements performed ondetected cells in connection with the radio RLF, a cell ID for the firstcell, a cell ID for the second cell, a cell ID for the third cell, and aHO failure type. Preferably, the RLF report further includes one or morein the group comprising: a RAT type, a carrier frequency, a locationarea code, and routing area codes for each cell in the system.

According to yet another embodiment of the disclosure the step oftransmitting involves: transmitting the information via one or more X2and/or S1 interfaces over one or more other intermediate BS, andpreferably the information is contained in a RLF indication message or ain a HO report message if the information is transmitted via one or moreX2 interfaces.

The behaviour for MS/MSs and BS/BSs are slightly different depending onthe error type, and therefore exemplary scenarios for the differenterror types will be outlined in the following description. The followingdescription in particular describes LTE RLF in connection with outgoingHO, i.e. from LTE to another RAT, but is not limited to this case whichis realised by the skilled person.

However, for LTE systems, the disclosure only impacts the LTE signallingfunctionality of the UE and the eNB; and further avoid any signallingfrom one RAT to another, e.g. by using the RIM procedure, therebyavoiding additional loading on the core network elements of both RATs.The UE must be able to log details of the RLF failure and be able toreport these when it finally attaches again to a LTE system. The impacton the 3GPP specifications is therefore lower than the prior artsolutions which is advantageous. Furthermore, the present method is alsoapplicable to RLFs or the equivalent in non-3GPP RATs such as WiMax orWi-Fi.

In the following disclosure exemplary embodiments of the cases: “IRATtoo late”, “IRAT too early”, and “IRAT wrong RAT” are described.However, as realised by the person skilled in the art the disclosure isnot limited to these embodiments, but also relates to and incorporatesall embodiments within the scope of the appended independent claims.

Exemplary Embodiment IRAT Too Late

-   1 A UE is connected to a cell 1, served by eNB1 supporting RAT1. The    UE moves out of coverage for cell 1, detects the radio problems and    tries to find a cell that it can connect to. After trying for a    certain time, the UE goes to idle mode.-   2 When in idle, the UE detects another cell (cell 2) belonging to a    different RAT (RAT2) then cell 1. The UE connects to this cell 2 and    stays there for some time (see (2) in FIG. 2).-   3 Eventually, the UE is moved back to RAT1 to another cell (cell n).    This could be cell 1 or a different cell. The UE remembers that it    has a stored a RLF report for the source cell in RAT1 that it could    send. The UE reports this to the eNB handling cell n on RAT1 (see    (3) in FIG. 2).-   4 The eNB serving cell n receives the RLF report, reads the report,    finds the cell ID (e.g. the EGCI) of the last serving cell (cell 1)    and if the cell is different (cell 1 not equal to cell n) forwards    the information to the eNB handling cell 1 (see (4) in FIG. 2).-   5 The source cell (cell 1) studies the RLF report and uses the    information to tune the mobility parameters, such as thresholds for    inter RAT mobility events.

Exemplary Embodiment IRAT Too Early

-   1 A UE is connected to cell 1, which is served by eNB1 supporting    RAT1.-   2 The UE is handed over to another cell (cell 2) belonging to    another RAT (RAT2). Within a short while, defined by a time    threshold (T_dwell), the UE detects radio problems and tries to find    a cell that it can connect to. After trying for a certain time, the    UE goes to idle mode (see (2) in FIG. 2).-   3 When in idle mode, the UE detects another cell (cell n) belonging    to RAT1. This could be cell 1 or a different cell. The UE connects    to this cell, and the UE realizes it has a RLF report for the source    cell in RAT1 that it could send. The UE reports this to the eNB    serving cell n on RAT1 (see (3) in FIG. 2).-   4 The eNB serving cell n receives the RLF report. The eNB reads this    RLF report, finds the EGCI of the last serving cell (cell 1) and if    the cell is different (cell 1 not equal to cell n) forwards the    information to the eNB serving cell 1 (see (4) in FIG. 2).-   5 The source cell (cell 1) studies the report and uses the    information to tune the mobility parameters.

Exemplary Embodiment IRAT Wrong RAT

-   1 A UE is connected to cell 1, served by eNB1 supporting RAT1.-   2 The UE is handed over to another cell (cell 2) belonging to    another RAT (RAT2). Within a short while, defined by a time    threshold (T_dwell), the UE detects radio problems and tries to find    a cell that it can connect to. After trying for a certain time, the    UE goes to idle mode (see (2) in FIG. 3).-   3 When in idle, the UE detects another cell (cell 3) belonging to    another RAT (RAT3).-   4 The UE connects to this cell (see (3) in FIG. 3). Eventually, the    UE is moved back to RAT1 to another cell (cell n). This could be    cell 1 or a different cell (see (4) in FIG. 3).-   5 The UE remembers that it has stored a RLF report for the source    cell in RAT1 that it could send. The UE reports this to the eNB    serving cell n on RAT1 (see (5) in FIG. 3).-   5 The eNB serving cell n receives the RLF report. The eNB reads this    RLF report, finds the EGCI of the last serving cell (cell 1) and if    the cell is different (cell 1 not equal to cell n) forwards the    information to the eNB serving cell 1. The source cell (cell 1)    studies the RLF report and uses the information to tune the mobility    parameters.

UE Behaviour

A UE has to detect and store the following cells in order to determinethe error type: the cell to which UE was connected to before the lastHO, the cell the UE was connected to before the RLF due to a HO failure,and the cell where the UE reconnects after the RLF.

In the exemplary embodiments illustrated above, unlike the intra LTEMRO, the UE itself indirectly partly identifies the error type bycomparing the time between HO and RLF with a threshold (T_dwell) todetermine whether the cell in which the RLF occurred should beconsidered as the source cell (in case of “IRAT too late”), or if thecell prior to the HO should be considered to be the source cell (in case“IRAT too early” or “IRAT wrong RAT”). This is needed, because the UEitself must identify the source cell to enable reporting to the correctRAT. Note that this requires that the threshold T_dwell is transmittedto the UE.

The following cells should be reported by the UE to support inter RATMRO:

-   -   Source cell—The cell supporting RAT1 to which the MS was        connected prior to the erroneous event.    -   Target cell—The cell to which a HO was attempted but where RLF        occurs either during HO, of shortly after.    -   Final cell—The cell where the MS manages to re-establish a        connection to the network.

The mapping between the detected cells and the reported cells for thedifferent error cases are listed in Table 1.

TABLE 1 Mapping between detected cells and reported cells Stored cellIRAT too late IRAT too early IRAT wrong RAT The cell a MS n/a Sourcecell Source cell was connected to before the last HO The cell a MSSource cell Target cell Target cell was connected to before the RLF Thecell where a Final cell Final cell Final cell MS reconnects after theRLF

It should be noted that it may be possible to use other definitions ofthe reported cells which would result in a different mapping. Further,it may also be possible for the UE to only detect the source cell toknow in which RAT to report and report all the detected cells as is [asis?], without any mapping to final/target cell.

It would also be possible for the UE to fully identify the error typeand explicitly signal this to the eNB. In order to distinguish between“IRAT too early” or “IRAT wrong RAT” the UE simply needs to compare ifthe source RAT is the same as the RAT where the UE manages to re-connectafter the RLF.

Inter RAT Information Collected and Reported by the UE

In addition to the contents of a RLF report according to prior art forinter RAT measurements, the UE may also according to an embodiment ofthe disclosure collect information about detected cells belonging toother RATs. The information should be of relevance for the source cellwhere the failure occurred, since the information eventually will betransferred to this cell.

The source cell is the cell in RAT1 which the UE was connected to priorto the erroneous event. Since an idea is to report this information tothe RAT which the source cell belongs to, the source cell can beuniquely identified by the global Identity (ID) for that RAT, forexample by the EGCI in LTE systems.

For the target and final cells, at least one of these will belong to adifferent RAT (otherwise, the event is not an inter RAT MRO event). Itmight be sufficient to refer to a certain inter RAT measurement objectID, used by the UE when measuring on other RATs, since this would pointto information specific for the neighbour RAT, e.g. carrier frequency.But, as this is configured per UE, and since the report from the UE maynot arrive immediately after the failure, it can not always be expectedthat this information will be kept in the eNB, so it may be moreappropriate to store the information in a self decodable way, byexplicitly signalling RAT type and carrier frequency in the system.

A similar discussion applies to the cell ID. Since the list of neighbourcells are transferred to the UE, it might be enough to just report theindex (to the neighbour cell list) of the cell where the UE manages toreconnect, but since the information may not be kept in the eNB, a cellID may be explicitly reported in the system according to an embodimentof the disclosure. Example of information that may be included is:

Carrier frequency E-UTRA Absolute Radio Frequency Channel Number(EARFCN).

RAT type, e.g. UTRAN/GERAN/CDMA2000.

Cell ID.

However, in some cases, the physical cell ID should be enough toidentify the target and final cells. One example of this is that thetarget cell is normally known in the source cell since the source eNBordered a HO to this cell. One possibility is therefore to only reportthe physical ID of the target and/or final cells according to anotherembodiment of the disclosure.

The final cell may however not be known by the source cell. And thetarget cell might have been removed from the Neighbour Relations Table(NRT) in the source cell. Therefore, another option is to always use theglobal ID of the final cell and/or for the target cell.

In case no mapping to target/final cell is performed by the UE (asmentioned as an alternative solution in the previous description) the UEwill instead need to report the detected cells, i.e.:

The cell the UE was connected to before the last HO.

The cell the UE was connected to before the RLF.

The cell where the UE reconnects after the RLF.

However, for this situation, the UE can not distinguish betweensource/target/final cells and will therefore need to report fullinformation for all cells, i.e.:

Carrier frequency EARFCN.

RAT type, e.g. UTRAN/GERAN/CDMA2000.

Cell ID.

As an additional improvement, the information can also be extended tominimize the need for UE Automatic Neighbour Relation (ANR)measurements. In that case, the information collected for target andfinal cells should always consist of CGI and the Routing Area Code (RAC)of the detected neighbour cell in case of GERAN detected cells, CGI, LACand, RAC in case of UTRAN detected cells and CGI in case of CDMA2000detected cells.

Yet another possibility is to use the information available in theMeasurement Object definition (as defined in 3GPP specificationTS36.331) to decide whether to report the above mentioned ANR relatedmeasurements. In LTE, there is a list of cells for which to report CGI(cellForWhichToReportCGI) for UTRAN and GERAN neighbour cells.

Expected Behaviour of eNB Receiving the Information from the UE

It is assumed that there is another solution in place enabling the UE toreport the RLF report also after the UE has gone back to idle.

One solution is that the eNB receiving the information from the UE readsthe source cell EGCI and in case this cell is handled by a differenteNB, forwards the information to the eNB handling the source cell. Thisforwarding can be accomplished by using the X2 interface and creating anew information element or by adding additional information to eitherthe existing RLF indication message or the RLF report message in LTEsystems. The additional information needed is the RAT information forthe target and final cell outlined above:

Carrier frequency.

RAT type.

Cell ID.

ANR related info (optional).

Another possibility is to forward the information via the S1 interface.

As an alternative solution, the receiving eNB may also forward theinformation not only to the eNB serving the source cell but also to theeNB serving the target/final cell, as it may be advantageous for thesecells to be aware of the error event.

In case no mapping is performed in the UE (as discussed above), or incase no explicit source cell is signalled in the message from the UE,the eNB itself must perform the error type detection to identify thesource cell, using the information reported by the UE. After this isdone, the eNB can forward the information to the source cell. In thiscase, the UE simply collects and reports the information about:

The cell the UE was connected to before the last HO.

The cell the UE was connected to before the RLF.

The cell where the UE reconnects after the RLF.

The time elapsed between the last HO and the RLF.

According to this embodiment the information regarding the cells shouldcontain RAT information (as defined above), and the UE only reports thiswhen managing to reconnect to a cell in the same RAT as it was connectedto before the RLF. The eNB receiving this information would then forwardit to the eNB serving the cell where the UE was connected before theRLF. This eNB can use the supplied information to determine the errortype and perform a similar mapping of detected and reported cells asdescribed above.

If no HO occurred shortly before the RLF, the error type is “IRAT toolate” and the cell where the UE was connected to before the RLF isactually the same as the source cell. Therefore, for this specific case,it is possible to directly use the detection of this error case toadjust inter RAT mobility parameters without requiring any additionalinformation transfer.

On the other hand, if a HO occurred a short time instance before theRLF, the error type would be either “IRAT too early” or “IRAT wrongRAT”; and the source cell would be the cell the UE was connected tobefore the last HO. In that case, information must be passed to thesource cell in a message similar to a HO report in intra-LTE MRO. Onedifference compared to current LTE MRO solution is that this message mayneed to be sent to another RAT and that the RAT information for allthree detected cells must be included. This solution is somewhat lesscomplex for the UE, but still requires new messaging across RAT in orderto enable detection of all error cases.

With reference to the above discussed embodiments of a method forproviding information in a cellular wireless communication system, thepresent disclosure also relates to a method in a MS and in a BS.

The method in a MS comprises the steps of: detecting a RLF while beingconnected to a first cell; re-establishing the connection in a secondcell served by a second BS; and providing information about the RLF onlyto cells supporting the same RAT as the first cell and/or a third cell,wherein the third cell is the cell to which the MS was connected beforethe first cell.

The method in a BS comprises the steps of: receiving a RLF reportrelating to a RLF for a MS while the MS is connected to a first cell;and providing information about the RLF to the first cell and/or to athird cell if the BS supports the same RAT as the first cell or thethird cell, wherein the third cell is the cell to which the MS wasconnected before the first cell.

According to an embodiment of the method above in a BS, the step ofproviding information involves: transmitting the information to a firstBS serving the first cell, or to a third BS serving the third cell,directly or via one or more X2 and/or S1 interfaces over one or moreother BSs in the communication system.

The method in a MS and in a BS above may be modified according todifferent embodiments of the method in a cellular wireless communicationsystem described in above.

Furthermore, as understood by the person skilled in the art, a method ina MS and a method in a BS according to the present disclosure may beimplemented in a computer having a processor connected to a computerreadable storage medium, the method runs in the computer causes theprocessor to execute the acts of the method. The computer program may bestored in the computer readable storage medium of a computer programproduct. The computer readable medium may consist of essentially anymemory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-OnlyMemory), an EPROM (Erasable PROM), a Flash memory, an EEPROM(Electrically Erasable PROM), or a hard disk drive.

Moreover, the disclosure also relates to a MS device and a BS device.The MS and BS devices may be any suitable devices in any relevant RATsystem applicable for the present disclosure such as LTE, UMTS,CDMA2000, WiMaX, Wi-Fi or GERAN. Hence, the MS may be a UE, MS or SS[SS?] and the BS may be eNB, NodeB or BTS.

The MS device is configured to: detect a RLF while being connected to afirst cell; re-establish the connection in a second cell served by asecond BS; and provide information about the RLF only to cellssupporting the same RAT as the first cell and/or a third cell, whereinthe third cell is the cell to which the MS was connected before thefirst cell.

The BS device is configured to: receive a RLF report relating to a RLFfor a MS while the MS is connected to a first cell; and provideinformation about said RLF to the first cell and/or to a third cell ifthe BS supports the same RAT as the first cell or the third cell,wherein the third cell is the cell to which the MS was connected beforethe first cell.

The MS device and BS device according to the disclosure may also bearranged according to the different embodiments of any of the methodsdisclosed.

1. A method for providing information in a cellular wirelesscommunication system, wherein each cell in said cellular wirelesscommunication system is served by a base station and supports a radioaccess technology (RAT) for radio communication between a cell and oneor more mobile stations connected to said cell; said cellular wirelesscommunication system employing a procedure in which a mobile station isallowed to be handed over from a cell to another cell supportingdifferent radio access technologies (RATs), and further employing aprocedure in which a mobile station suffering from a radio link failure(RLF), when being connected to a cell, is allowed to attempt tore-connect to another cell supporting a different radio accesstechnology (RAT), the method comprising: detecting a radio link failure(RLF) for a mobile station while connected to a first cell;re-establishing the connection in a second cell; and providinginformation about said radio link failure (RLF) only to cells supportingthe same radio access technology (RAT) as said first cell and a thirdcell, wherein said third cell is the cell to which said mobile stationwas connected before said first cell.
 2. The method according to claim1, wherein said information comprises a radio link failure (RLF) reportproduced by said mobile station.
 3. The method according to claim 2,wherein said radio link failure (RLF) report includes one or more in thegroup comprising: measurements performed on detected cells in connectionwith said radio link failure (RLF), a cell identity (ID) for said firstcell, a cell identity (ID) for said second cell, a cell identity (ID)for said third cell, and a handover failure type.
 4. The methodaccording to claim 2, wherein said information is transmitted by saidmobile station to a fourth base station serving a fourth cell to whichsaid mobile station is connected after said radio link failure (RLF), ifsaid fourth cell supports the same radio access technology (RAT) as saidfirst and third cells, wherein said fourth cell is any of said first,second or third cells or a different cell.
 5. The method according toclaim 4, further comprising: transmitting by said fourth base stationsaid information to a first base station serving said first cells. 6.The method according to claim 5, wherein said transmitting involves:transmitting said information via one or more X2 interfaces in saidcellular wireless communication system.
 7. The method according to claim6, wherein said information is contained in a radio link failure (RLF)indication message or a in a handover report message.
 8. The methodaccording to claim 1, wherein the re-establishing the connection in thesecond cell fails.
 9. The method according to claim 8, furthercomprising connecting to a fifth cell supporting different radio accesstechnology (RAT) from the first and third cell after the re-establishingthe connection in the second cell fails.
 10. A method in a base stationfor providing information in a cellular wireless communication system,wherein each cell in said cellular wireless communication system isserved by a base station and supports a radio access technology (RAT)for radio communication between a cell and one or more mobile stationsconnected to said cell; said cellular wireless communication systememploying a procedure in which a mobile station is allowed to be handedover from a cell to another cell supporting different radio accesstechnologies (RATs), and further employing a procedure in which a mobilestation suffering from a radio link failure (RLF), when being connectedto a cell, is allowed to attempt to re-connect to another cellsupporting a different radio access technology (RAT), the methodcomprising: receiving a radio link failure (RLF) report relating to aradio link failure (RLF) for a mobile station while said mobile stationis connected to a first cell if said base station supports the sameradio access technology (RAT) as said first cell and a third cell; andproviding information about said radio link failure (RLF) to said firstcell if said base station supports the same radio access technology(RAT) as said first cell and said third cell, wherein said third cell isthe cell to which said mobile station was connected before said firstcell.
 11. The method in a base station according to claim 10, whereinsaid providing information involves: transmitting said information to afirst base station serving said first cell, via one or more X2interfaces in said cellular wireless communication system.
 12. Themethod according to claim 11, wherein said information is contained in aradio link failure (RLF) indication message or a in a handover reportmessage.
 13. The method according to claim 10, wherein said informationcomprises a radio link failure (RLF) report produced by said mobilestation.
 14. The method according to claim 13, wherein said radio linkfailure (RLF) report includes one or more in the group comprising:measurements performed on detected cells in connection with said radiolink failure (RLF), a cell identity (ID) for said first cell, a cellidentity (ID) for said second cell, a cell identity (ID) for said thirdcell, and a handover failure type.
 15. A mobile station device forproviding information in a cellular wireless communication system,wherein each cell in said cellular wireless communication system isarranged to be served by a base station and supports a radio accesstechnology (RAT) for radio communication between a cell and one or moremobile stations connected to said cell; said cellular wirelesscommunication system employing a procedure in which a mobile station isallowed to be handed over from a cell to another cell supportingdifferent radio access technologies (RATs), and further employing aprocedure in which a mobile station suffering from a radio link failure(RLF), when being connected to a cell, is allowed to attempt tore-connect to another cell supporting a different radio accesstechnology (RAT); the mobile station device comprising a processorconfigured to: detect a radio link failure (RLF) while being connectedto a first cell; re-establish the connection in a second cell; andprovide information about said radio link failure (RLF) only to cellssupporting the same radio access technology (RAT) as said first cell anda third cell, wherein said third cell is the cell to which said mobilestation was connected before said first cell.
 16. The mobile stationdevice according to claim 15, wherein said information comprises a radiolink failure (RLF) report produced by said mobile station.
 17. Themobile station device according to claim 16, wherein said radio linkfailure (RLF) report includes one or more in the group comprising:measurements performed on detected cells in connection with said radiolink failure (RLF), a cell identity (ID) for said first cell, a cellidentity (ID) for said second cell, a cell identity (ID) for said thirdcell, and a handover failure type.
 18. The mobile station deviceaccording to claim 16, wherein said information is transmitted by saidmobile station to a fourth base station serving a fourth cell to whichsaid mobile station is connected after said radio link failure (RLF), ifsaid fourth cell supports the same radio access technology (RAT) as saidfirst and third cells, wherein said fourth cell is any of said first,second or third cells or a different cell.
 19. The mobile station deviceaccording to claim 15, wherein the re-establishing the connection in thesecond cell fails.
 20. The mobile station device according to claim 19,further comprising unit configured to connect to a fifth cell supportingdifferent radio access technology (RAT) from the first and third cellafter the re-establishing the connection in the second cell fails.
 21. Abase station device for providing information in a cellular wirelesscommunication system, wherein each cell in said cellular wirelesscommunication system is arranged to be served by a base station andsupports a radio access technology (RAT) for radio communication betweena cell and one or more mobile stations connected to said cell; saidcellular wireless communication system employing a procedure in which amobile station is allowed to be handed over from a cell to another cellsupporting different radio access technologies (RATs), and furtheremploying a procedure in which a mobile station suffering from a radiolink failure (RLF), when being connected to a cell, is allowed toattempt to re-connect to another cell supporting a different radioaccess technology (RAT), the base station device comprising a processorconfigured to: receive a radio link failure (RLF) report relating to aradio link failure (RLF) for said mobile station while said mobilestation is connected to a first cell if said base station supports thesame radio access technology (RAT) as said first cell and a third cell;and provide information about said radio link failure (RLF) to saidfirst cell if said base station supports the same radio accesstechnology (RAT) as said first cell and said third cell, wherein saidthird cell is the cell to which said mobile station was connected beforesaid first cell.
 22. The base station device according to claim 21,wherein the processor is configured to transmit said information to afirst base station serving said first cell, via one or more X2interfaces in said cellular wireless communication system.
 23. The basestation device according to claim 22, wherein said information iscontained in a radio link failure (RLF) indication message or a in ahandover report message.
 24. The base station device according to claim21, wherein said information comprises a radio link failure (RLF) reportproduced by said mobile station.
 25. The base station device accordingto claim 24, wherein said radio link failure (RLF) report includes oneor more in the group comprising: measurements performed on detectedcells in connection with said radio link failure (RLF), a cell identity(ID) for said first cell, a cell identity (ID) for said second cell, acell identity (ID) for said third cell, and a handover failure type.